CMSC 132 Object Oriented Programming II Java Inner Classes Department of Computer Science University of Maryland College Park 1 Overview Classes Top level vs inner nested Inner classes Iterator example Used inside outer class Anonymous inner classes Syntax Uses for GUIs Nested classes 2 Java Classes Top level classes Declared inside package Visible throughout package perhaps further Normally declared in their own file Public classes must be defined in their own file Not required for other classes Inner and nested classes Declared inside class or method Normally used only in outer enclosing class Can have wider visibility 3 Inner Nested Classes Inner class Nested class Anonymous inner class Examples public class MyOuterClass public class MyInnerClass static public class MyNestedClass Iterator iterator return new Iterator 4 Inner Classes Description Class defined in scope of another class May be named or anonymous Useful property Outer inner class can directly access each other s fields methods even if private 5 Inner Classes Example public class OuterClass private int x private class InnerClass private int y void foo x 1 access private field void bar InnerClass ic new InnerClass ic y 2 access private field 6 Inner Class Link To Outer Class Inner class instance Has association to an instance of outer class Must be instantiated with an enclosing instance Is tied to outer class object at moment of creation can not be changed MyList MyIterator MyIterator MyList MyIterator 7 Inner Classes Useful for Private helper classes Logical grouping of functionality Data hiding Linkage to outer class Inner class object tied to outer class object Examples Iterator for Java Collections ActionListener for Java GUI widgets 8 Iterator Example MyList public class MyList private Object a private int size Want to make MyList implement Iterable Skipping generic types at the moment Need to be able to return an Iterator 9 Problematic MyIterator Design public class MyIterator implements Iterator private MyList list private int pos MyIterator MyList list this list list pos 0 public boolean hasNext return pos list size public Object next return list a pos 10 MyIterator Design Problems Need to maintain reference to MyList Need to access private data in MyList Solution Define MyIterator as inner class for MyList Instance of MyIterator tied to instance of MyList MyIterator methods can access private MyList fields MyIterator objects can iterate through elements of MyList 11 Successful MyIterator Design Code public class MyList implements Iterable private Object a private int size public Iterator iterator return new MyIterator public class MyIterator implements Iterator private int pos 0 public boolean hasNext return pos size public Object next return a pos 12 Instantiating Inner Class How to access instance of inner class Common gimmick Outer class method returns instance of inner class Used by Java Collections Library for Iterators Example code public class MyList public class IC implements Iterator public Iterator iterator return new IC creates instance of IC MyList m new MyList Iterator it m iterator 13 Using Inner Class Inside Outer Class Assume class OC defines an inner class IC Inside methods of OC just use IC normally Use a new IC to create instances of IC a is associated with the specific IC object referenced by this 14 Using Inner Class Inside Outer Class Code public class OC outer class private int x 2 private class IC inner class private int y 4 private int getSum return x y void bar IC z new IC create new IC assoc w this z getSum treat z like normal object 15 Accessing Outer Scope Code public class OC outer class int x 2 private class IC inner class int x 6 private void getX inner class method int x 8 System out println x prints 8 System out println this x prints 6 System out println OC this x prints 2 16 Method Invocations Method invocations on inner class Can be transparently redirected to outer instance Resolving method call on unspecified object 1 See if method can be resolved on inner object 2 If not see if method can be resolved on corresponding instance of outer object 3 If nested multiple levels keep on looking 17 Anonymous Inner Class Description Inner class without name Defined where you create an instance of it In the middle of a method Returns an instance of anonymous inner class Useful if the only thing you want to do with an inner class is create instances of it in one location Syntax new ReturnType unnamed inner class body of class implementing ReturnType 18 Anonymous Inner Class Code public class MyList public Iterator iterator return new Iterator unnamed inner class implementing Iterator MyList m new MyList Iterator it m iterator 19 Example Anonymous Inner Classes Use new Foo public int one return 1 public int add int x int y return x y To define an anonymous inner class that Extends class Foo implements interface Foo Defines methods one add 20 MyList With Explicit Inner Class Code public class MyList implements Iterable private Object a private int size public Iterator iterator return new MyIterator public class MyIterator implements Iterator private int pos 0 public boolean hasNext return pos size public Object next return a pos 21 MyList With Anonymous Inner Class Code public class MyList implements Iterable private Object a private int size public Iterator iterator return new Iterator private int pos 0 public boolean hasNext return pos size public Object next return a pos 22 Support For Java GUIs Graphical User Interface GUI Java AWT Swing libraries Event driven programming model Components may generate events E g ActionEvent KeyEvent MouseEvent Requires event listeners to handle event Event listeners frequently implemented using anonymous classes Used only in one location Implements event listener interfaces 23 Using Inner Classes in GUIs javax swing SwingUtilities invokeLater new Runnable public void run createAndDisplayGUI button addActionListener new ActionListener public void actionPerformed ActionEvent evt System out println Button pushed 24 Nested Class Description Similar to inner class but declared as static class No link to an instance of the outer class Can directly access outer class fields methods Useful if inner class object Associated with different outer class objects Survives longer than outer class object Example class LinkedList static class Node Node next Node head 25 Summary of Inner Nested Classes All inner nested classes Defined inside another class Can
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